Rugby league is considered to be the ultimate television sport by many in the industry, both back then and now. Nearly all of the action is concentrated to a small portion of the field at any given time - the area surrounding the play - and it can be easily captured by a television camera.
Aussie rules, by contrast, has a huge amount of action happening off the ball which doesn't particularly translate well to television. When the play is concentrated to a narrow area it is usually a ruck which is quite messy and chaotic and not all that enjoyable to watch.
Furthermore, whilst the AFL did indeed have a broader national footprint at the time, several of their expansion teams were basket-cases with the only club doing really well being the West Coast Eagles. Compare that to the ARL who had two of its expansion clubs dominating the competition in the Canberra Raiders and Brisbane Broncos.
Additionally, the ARL completely dominated Australia's most populous and financially powerful city in Sydney.